Grid Grounding Calculations for a 132-KV Substation Using Soil Backfilling

Abstract

Safe and reliable grid design is complicated by the compact design of gas insulated substations (GIS). This paper discusses the complete grounding design procedure for a 132-KV substation in the city of Al Kharj based on IEEE-80 specifications and local standard practices in the Kingdom of Saudi Arabia. To ensure a safely grounded grid design, local Saudi Electricity Company (SEC) standard design constraints were used. Current Distribution Electromagnetic Interference, Grounding and Soil Structure (CDEGS) software was used to compare the test results for soils with and without backfilling. Due to the high resistivity of the plot, soil with a low resistivity should be used as backfill. The calculated results were compared to the standard values of ground potential rise, step voltage, touch voltage and soil resistance. This paper contributes to an understanding of effective grounding techniques for soil with high resistivity and optimizes the increase in cost due to backfilling.

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Date of publication xxxx 00, 0000, date of current version xxxx 00, 0000.
Digital Object Identifier 10.1109/ACCESS.2017.Doi Number
Grid Grounding Calculations for a 132-KV
Substation Using Soil Backfilling
A. Saeed Alyami
1
, Member, IEEE
1
Electrical Engineering Department, Majmaah University, Almajmaah 11592, Saudi Arabia, email: s.alyami@mu.edu.sa.
This work was supported in part by Deanship of scientific research at Majmaah University under project 1440-138.
ABSTRACT
Safe and reliable grid design is complicated by the compact design of gas insulated substations
(GIS). This paper discusses the complete grounding design procedure for a 132-KV substation in the city of Al
Kharj based on IEEE-80 specifications and local standard practices in the Kingdom of Saudi Arabia. To ensure
a safely grounded grid design, local Saudi Electricity Company (SEC) standard design constraints were used.
Current Distribution Electromagnetic Interference, Grounding and Soil Structure (CDEGS) software was used
to compare the test results for soils with and without backfilling. Due to the high resistivity of the plot, soil with
a low resistivity should be used as backfill. The calculated results were compared to the standard values of
ground potential rise, step voltage, touch voltage and soil resistance. This paper contributes to an understanding
of effective grounding techniques for soil with high resistivity and optimizes the increase in cost due to
backfilling.
INDEX TERMS Grounding grids, CDEGS, Step voltage, Touch voltage, Ground potential rise
I. INTRODUCTION
Grid grounding is an established technique used to ensure
the safety of grid equipment and individuals. Over time, air-
insulated substations have been upgraded to GIS
substations. The compact size of substations and the
enhanced voltage levels due to increments in load
consumption make the substations vulnerable to electric
shocks. Grounding grids are necessary to achieve minimal
impedance values and enable fault currents to flow easily
toward the ground, thereby limiting potential surges to
substation equipment and promptly clearing all types of
transient surges. In general, ground grid impedance should
be less than 1–5 [1]. Moreover, grounding a grid allows
the installed equipment and protective devices to perform
properly and safely. For electrical equipment, such as
power transformers, capacitor banks, reactors, and auxiliary
station transformers, safety and reliability are top priorities.
To achieve stability for such equipment, neutral point
grounding is essential. The integrity of the grounding grid
under both normal and faulty conditions enables the
continuity of service and ensures personal safety in a
facility by limiting the danger of electric shock. To achieve
reliable and cost-effective grid grounding, several factors
should be considered. Based on IEEE-80 specifications, the
CDEGS [2] software tool was used for analysis.
A new 132-KV substation called Al-Tawdihiyah tag# S/S
8725 has been designed for the city of Al Kharj, which is
south of the capital city in the Kingdom of Saudi Arabia.
The objective of this paper is to investigate and test the soil
data provided by the Saudi Electricity Company for
effective grounding. Soil backfilling was used due to high
soil resistivity to achieve improved and reliable grid
grounding resistance while minimizing the cost of
backfilling. Multilayer soil was used to approximate a
highly nonuniform soil, which requires complex
calculations with a computer program or graphical methods.
All the design constraints considered in this paper are based
on local standard practice in the Kingdom of Saudi Arabia.
For grid equipment safety, a pure conductive copper
connection with the grounding grid is needed, as shown for
a typical auxiliary transformer and ring main unit in Fig. 1.
For this purpose, compression lugs, grounding rods, and a
bare, soft drawn, annealed, stranded copper conductor with
proper thermo welds or compression joints are necessary.
FIGURE 1.
General Arrangement for Equipment Grounding

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During design, the resistance between personnel and the
ground should be high enough to prevent any type of
current from flowing through the human body [3].
II. METHODOLOGY
Grid grounding mainly relies on proper grid conductor
sizing that can achieve permissible touch and step voltages
and grid potential rise relative to the grounding joints,
which is considered to be the potential of remote earth.
During the construction phase of the substation, the criteria
defined in the IEEE-80 publication “Guide for Safety in
Substation Grounding” were followed to protect personnel
and equipment from electric shocks. The local standards of
the Saudi Electricity Company were followed as the design
constraints to develop an effective grid ground framework
in the Kingdom of Saudi Arabia [4]. Effective grid
grounding helps to protect telecom and control equipment
[4].
A. GRID CONDUCTOR SIZING
Soft drawn, stranded copper should be used for the ground
grid conductors. The conductor should be round to
maximize cross-sectional contact with the ground. In
coastal zones with low soil resistivity, a tinned copper
conductor should be used. Copper-clad steel should be used
for ground rods to resist fusing and prevent the electric
joints under the severe conditions due to the increase in
fault-current and fault period to which it might be exposed.
The grounding conductor should be composed of soft-
drawn, annealed copper. The required cross-sectional area
is calculated based on IEEE std. 80 Eq. 30. The formula for
calculating the Cu conductor area is given by




















a
m
rrc
f
TK
TK
t
TACP
I
0
0
4
mm
ln
10
A
2

(1)
where
tc = Maximum possible clearing, taken as 1.0 sec.
r = Thermal coefficient of resistivity of the conductor
material at the reference temperature
Tr = 0.00393
r = Resistivity of the ground conductor at the
reference temperature Tr in µ-cm = 1.72
TACP = Thermal capacity [4] J/cm
3
.°C = 3.42
Tm = Fusing temperature in °C = 1083
Ta = Ambient temperature in °C = 50
k0 = 1/ 0 or k0 = (1/ r) - Tr = 234 [5].
B.
TOLERABLE TOUCH VOLTAGE
Touch voltage is the voltage difference created between any
electrically conductive equipment and a standing person
who contacts that equipment. A person may contact
equipment by touching it with both hands or with one hand
while their feet are resting on the ground. Ideally, the
ground voltage would be zero volts, creating a potential
gradient during current leakage or fault current (i.e., the
amount of current that flow through electrical equipment
during electrical fault condition) [5].
ts sCs 116.0)5.11000(
Vt 


(2)
where
Cs = Reduction factor for derating the nominal value of
surface layer resistivity; it is 1 when there is no protective
surface layer (protective layer resistivity equal to soil
resistivity). For a protective surface layer with a resistivity
higher than soil resistivity, Cs is < 1.
ts = Duration of the shock current in sec, which usually
ranges from 0.5 to 1.0 sec. For SEC applications, this value
will be taken as 0.5 sec or the back-up clearing time,
whichever is greater.
s = Resistivity of the surfacing material in ohm-meters,
which ranges from 1000 to 5000 in value [5].
The fault hazard analysis for achieving a permissible touch
voltage limit is also defined by EN 50522 [6].
C.
TOLERABLE STEP VOLTAGE
Step voltage Vs is the difference in surface potential that
could be experienced by a person bridging a distance of 1 m
with their feet without contacting any grounded object.
ts
sCs 116.0)61000(
Vs 


(3)
Here,
Cs = Reduction factor for derating the nominal value of
surface layer resistivity; it is 1 when there is no protective
surface layer (protective layer resistivity equal to soil
resistivity). For a protective surface layer with a resistivity
higher than soil resistivity, Cs is < 1.
ts = Duration of the shock current in sec, which usually
ranges from 0.5 to 1.0 sec. For SEC applications, this value
will be taken as 0.5 sec or the back-up clearing time,
whichever is greater.
s = Resistivity of the surfacing material in ohm-m, which
ranges from 1000 to 5000 in value [5]
The influences of step and touch voltages vary with body
impedance and shoes, as considered in [7].
D.
GROUND POTENTIAL RISE
The maximum electrical potential that a ground electrode
may attain relative to a distant grounding point is assumed
to be the potential of remote earth. This voltage, GPR, is
equal to the maximum grid current multiplied by the grid
resistance [5]
RgIGGPR 
(4)
where
IG = Maximum grid current in amperes
Rg = Grid resistance in ohms [5]
The value of substation grounding resistance is calculated
using the following formula:

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)]
201
1
1(
20
11
[Rg AhA
Lt 


(5)
where
Rg = Substation ground resistance in ohms
 = Average ground resistivity in ohm-m
A = Area occupied by the ground grid in m²
Lt = Total buried length of conductors in m (this value is a
combination of the grid rod length and the combined length
of the earthing conductor and ground rods).
h = Depth of the grid in meters, excluding any asphalt
covering [5]
Once the grounding mat has been laid along with the
desired grounding electrodes, all ancillary grid equipment is
bonded. The grid grounding conductor is determined
through an iterative process using CDEGS software. (5)
plays a vital role in the cost estimation of the total grid
conductor length. Effective secondary protections will
enhance substation reliability and grid life.
III. GRID DESIGN CALCULATIONS
Resistivity governs the amount of current that passes
through a material when a specific potential difference is
applied. The following equation is used to determine the
average soil resistivity to a depth equal to the distance
between the electrodes:
aR

2
(6)
where
p = Average soil resistivity
 = 3.1416
a = Distance between electrodes
R = Test instrument resistance reading in ohms [5].
Table 1 shows the earth resistivity test (ERT) measurements
of electrode resistance and soil resistivity. The electrical
resistivity test was conducted at the site for sixteen test
points according to IEEE-81 in eight directions. The test
was performed using a Terrameter SAS 1000 device with
the 4-electrode Wenner array method.
TABLE 1. EARTH RESISTIVITY TEST VALUES USING WENNER
METHOD
SOIL
RESISTIVITY
(ohm-m)
1.5 3 4.5 6 9 15
ERT-1 113 142 169 230 281 302
ERT-2 102 127 176 218 251 273
ERT-3 108 129 170 221 267 280
ERT-4 83 108 168 214 261 281
ERT-5 150 219 287 332 357 356
ERT-6 187 254 328 341 374 360
ERT-7 130 190 256 320 316 322
ERT-8 144 208 274 317 357 340
ERT-9 121 189 252 289 335 362
ERT-10 169 226 292 327 384 373
ERT-11 144 193 256 308 335 347
ERT-12 116 157 221 272 327 336
ERT-13 86 133 186 231 265 287
ERT-14 133 174 245 307 340 341
ERT-15 121 167 218 283 336 341
ERT-16 127 182 223 274 318 350
AVERAGE
(ohm-m) 127 175 233 280 319 328
Initially, the soil structure of the allocated substation plot
was studied using the Wenner four point method [8] to
gather earth resistivity (ohm-meter) data. The electrical
resistivity test procedure uses a controlled current produced
artificially between two electrodes implanted in the ground
as the energy source. Another pair of electrodes measures
the potential difference produced as a result of this current
flow.
From the data obtained by varying the spacing and
distribution of the electrodes, it is possible to compute the
apparent resistivity of the ground. This parameter has been
normalized over a uniform subsurface; it is independent of
current input, electrode arrangement and spacing. The
Wenner array was used at this site. The field procedure
consists of taking a succession of apparent resistivity
readings with increasing electrode spacing. The method
relies on the fact that the larger the spacing between the
current and potential electrodes, the greater the depth of
investigation. Similarly, an integrated methodology ensures
the protection of personnel and grid equipment against HV
short circuits by providing an optimized and economical
grounding system, as discussed in [9].
For the purpose of investigation, the method of varying
electrode spacing was used. The array must always retain
symmetry about a certain point, which is also an effective
point of observation. The instrument used was a Geppulse
Megger Digital Multi Earth Tester Serial No. 156
manufactured in Finland. This instrument, which is battery
powered and has a maximum of 12 volts available,
measures a minimum resistance of 0.001 ohms. The current
electrodes were 85-cm-long steel stakes that can be easily
driven into the ground. Table 2 shows the tabulated soil
resistivity values, which are to be used for soil modeling.
The probe direction was considered in four possible
directions. The abbreviations used in Table 2 are defined
below.
NS = North-South
EW = East-West
NW/SE = Northwest-Southeast
NE/SW = Northeast-Southwest
TABLE 2. EARTH RESISTIVITY TEST VALUES USING THE
WENNER METHOD

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Probe Direction
& Distance
Resistivity (ohm-meter)
1.5 3
5 8 12 15
SR
1
NS 96 144 160 213 175 154
EW 139 226 210 260
NW/SE 132 226 262 306 326
NE/SW 155 196 246 281 286 262
SR
2
NS 120 158 253 270 283 332
EW 113 157 216 265 291 362
NW/SE 112 174 230 221 297 360
NE/SW 111 157 223 285 358 351
SR
3
NS 130 190 271 264 340
EW 125 222 202 283 323 338
NW/SE 126 193 202 295 352 328
NE/SW 124 214 234 285 341 336
SR
4
NS 167 275 303 323 381 412
EW 204 270 275 313 401 407
NW/SE 164 263 253 309 392 406
NE/SW 172 263 272 320 409 439
Average value = 140.0975 140.0975 140 210 245 286
The earth resistivity test values are used to compute the soil
model in CDEGS. The resistivity of the upper layer is 66.47
ohm-m, and the resistivity of the lower layer is 370.62 ohm-
m. Different locations showed that the presence of high-
resistivity patches made safe grounding difficult, as shown
below in Fig. 2.
FIGURE 2.
Soil Resistivity Model Without Backfilling
Due to high resistivity soil, low resistivity soil was used for
backfilling to control soil grid resistance. As shown in Fig.
2, the resistivity of the upper layer is reduced to 123.59
ohm-m, and the resistivity of the lower layer is reduced to
329.28 ohm-m by using 8 probes installed at different
locations. As per actual site conditions, the complete
substation yard area was backfilled with soil at an average
thickness of 1.8 m. In actual conditions, the grounding grid
should be laid within the backfill soil.
FIGURE 3.
Soil Resistivity Model with Backfilling
Calculations were then performed with the soil model
considering the top layer as backfill material with a low
resistivity of 62.4 ohm-m and a thickness of 4 m, the
second layer with a resistivity of 123.59 ohm-m and a
thickness of 1.67 m and the final layer with a resistivity of
329.28 ohm-m and an infinite depth from the soil model, as
shown in Fig. 4.
FIGURE 4.
Sectional view for soil resistivity with backfilling material
The grounding grid should cover the protected area within
the substation boundary and should extend at least 1.5 m
outside the substation boundary on all sides. The grounding
grid should be buried at a depth ranging from 0.5 to 1.5 m
[4] below the final grade (excluding asphalt covering). The
spacing of the main conductors generally ranges from 3 to
15 m [4]. In congested areas, reduced intervals may be
desirable. Grid spacing should be halved around the
perimeter of the grid to reduce periphery voltage gradients.
Reinforcement bars in concrete slabs, foundations and duct

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banks should be connected to the grounding grid using the
appropriate thermo weld joints. The main conductors and
secondary conductors should be bonded at points of
crossover by thermo welds. This connection is normally
achieved with a grid of horizontally buried conductors and
is supplemented by a number of vertical rods connected to
the grid.
Ground rods should have minimum dimensions of 19 mm
 × 3.6 m [4], and the size should be selected for the
breaker short circuit rating. For two-layer and multilayer
soil models in which the upper layer has high soil
resistivity, deep driven rods should be considered so that
the rod is in contact with the low-resistivity lower soil
layer. Prior to backfilling, the ground rods should be
installed. As per recommended engineering practice, solid
ground rods are used in this study [10].
A soft-drawn, annealed copper conductor with a cross-
sectional area of 150 mm
2
is selected (2). A 100×100 m
mesh, as shown in Fig. 5, is laid 0.5 m below the asphalt
level. For grounding grid design in the KSA, the soil
resistivity of asphalt should be considered to be 3000 ohm-
m at a depth of 0.1 m.
FIGURE 5.
Auto Grid Pro: Main Grid Design (Top View)
Design parameters are considered based on the Saudi
Electricity Company [4]. The symmetrical ground fault
current (I
f
) is considered to be 40 kA for the system, and
the X/R ratio equals 20. The current division factor (S
f
) is
0.7. The time of current flow (tc) during the duration of
shock (ts) is 1 sec. It is assumed that the fault current
includes any conceivable system additions over the next 25
years. Thus, no additional safety factor for system growth is
added; that is, Cp = 1. The safety factor for a body to bear
the shock of a current for the majority of people (weighing
approximately 50 kg) is calculated by the following
formula:
ts
I
B
116.0

(7)
where
B
I
= rms magnitude of tolerable shock current through the
body in amperes.
ts = Duration of the current exposure in sec (shock
duration). [5]
Possible destruction due to transient and fault current
could be encountered due to different possibilities, as
discussed below.
The first, second and asphalt layers have resistivities of
66.47 ohms, 370 ohms and 3000 ohms, respectively. A total
of 28 pure copper steel clad rods with diameters of 19 mm
were buried at a vertical depth of 3.6 m. After placement of
the rods, the touch voltage was calculated by using (2). The
calculated touch voltage V
t1
of 470.34 V was greater than
the permissible touch voltage V
t2
of 468.8 V. The touch
voltage with the variant grid potential modeling is shown
below for all 2D spots and 3D spots in Figs. 6 and 7,
respectively.
FIGURE 6.
Touch Voltage (All-2D Spots) without Backfilling
FIGURE 7.
Touch Voltage (All-3D Spots) without Backfilling
The touch voltage results did not satisfy the grid safety
requirements, and a large touch voltage value was found at

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the grid periphery, which was addressed by backfilling soil
in the high-resistance plot areas. The first, second and
asphalt layers have resistivities of 62.4 ohms, 123.59 ohms
and 3000 ohms, respectively.
A total of 28 pure copper steel clad rods with diameters
of 19 mm were buried at a vertical depth of 3.6 m. After
placement of the rods, the touch voltage was calculated by
using (2). The calculated touch voltage V
t1
of 466.8 V was
less than the permissible touch voltage V
t2
of 468.8 V. The
touch voltage obtained by variant grid potential modeling
for backfilling with a low-resistivity soil is shown for all 2D
spots and 3D spots in Figs. 8 and 9, respectively.
FIGURE 8.
Touch Voltage (All-2D Spots) with Backfilling
FIGURE 9.
Touch Voltage (All-3D Spots) with Backfilling
The first, second and asphalt layers have resistivities of
66.47 ohms, 370 ohms and 3000 ohms, respectively. A total
of 28 pure copper steel clad rods with diameters of 19 mm
were buried at a vertical burial depth of 3.6 m. After
placement of the rods, the step voltage was calculated by
using (3). The calculated step voltage V
s1
of 585.64 V was
greater than the permissible touch voltage V
s2
of 500 V. The
step voltage obtained by variant grid potential modeling is
shown for all 2D spots and 3D spots in Figs. 10 and 11,
respectively.
FIGURE 10.
Step Voltage (All-2D Spots) without Backfilling
FIGURE 11.
Step Voltage (All-3D Spots) without Backfilling
The step voltage results did not satisfy the grid safety
requirements, and a large touch voltage value was found at
the grid periphery, which was addressed by backfilling soil
in the high-resistance plot areas. The first, second and
asphalt layers have resistivities of 62.4, 123.59 and 3000
ohms, respectively.
A total of 28 pure copper steel clad rods with diameters
of 19 mm were buried at a vertical burial depth of 3.6 m.
After placement of rods, the step voltage was calculated by
using (3). The calculated step voltage V
s1
of 475.78 V was
less than the permissible touch voltage V
s2
of 500 V. The
step voltage obtained by variant grid potential modeling for
backfilling with a low-resistivity soil is shown for all 2D
spots and 3D spots in Figs. 12 and 13, respectively.

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FIGURE 12.
Step Voltage (All-2D Spots) with Backfilling
FIGURE 13.
Step Voltage (All-3D Spots) with Backfilling
GPR should be restricted as much as possible to safeguard
microprocessor-based equipment and communication
equipment [4].
The proposed grid has a grid spacing of 3 m, which
gradually decreases at the mesh periphery. A total of 28
grounding rods were used at different locations, with each
rod having a length of 3.6 m and a diameter of 1.9 cm.
Table 2 shows the grid grounding design comparisons for
multilayer soil resistivity, touch voltage, step voltage, grid
resistance, total buried length of conductor and ground
potential rise before and after soil filling.
Safe and permissible design requirements have been
obtained. All equipment with a metallic body should be
connected with compressed thermo weld lugs, and ground
rods for power transformers, auxiliary transformers,
lightning masts, surge arresters, etc. should be added to
complete the grid design details.
Comparative results for the final values from CDEGS are
summarized in Table 3.
Table 3. Comparison of Grounding Grid Design Final Parameters
Sr# Parameters Values Without
Backfilling
Values With
Backfilling
1
Soil resistivity layer #01
(ohm-m) Infinity Infinity
2 Soil resistivity layer #02
(ohm-m) 66.47 62.4
3 Soil resistivity layer #03
(ohm-m) 370.42 123.59
4 Touch Voltage (V) 470.34 466.8
5 Step Voltage (V) 475.78 585.64
6 Grid Resistance (ohm) 0.97 0.75
7 Total Buried Electrode
(m) 8500 13261
8 GPR (V) 27279 21132
Grounding rods were fixed per the grid ancillary
equipment. The grounding grid spacing was decreased at
the grid periphery, and the length of the buried conductors
was increased to lower grid resistance by 64%. As a result,
the increase in buried length was constrained to achieve
safe grid substation and restricted increase in cost. The
desired effective grounding details of the rods used are
listed in Table 4.
Table 4. Details of Rods for Grid Grounding
Sr# Parameters Quantity Without
Backfilling
Quantity With
Backfilling
1 Rods for Main Grid 28 28
2 Rods for Power
Transformer 18 18
3 Rods For Auxiliary
Transformer 02 02
4 Rods For Surge Arrestor
& Capacitor bank 12 12
5 Rods For Test Pits 12 12
6 Rods For Lightening
Mast 3 3
7 Rods For Building 6 6
In Table 4, the rods for the grid grounding have been kept
the same before and after backfilling from the simulation of
the backfilling soil. The new soil results showed that rods
had not increased from the old soil. Thus, the cost has been
restricted to avoid the increase in the rods.
IV. CONCLUSION
In this paper, we investigated an effective grid grounding
technique. Based on the available land for the allocated
132-KV substation, the soil resistivity was high enough to
increase the ground potential. Due to an abnormal increase
in the voltage gradient, the functionality of telecom and
other control devices is expected to be compromised.

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Furthermore, the grid potential rise could be reduced by
treating the high-resistivity soil patches.
Backfilling with low-resistivity material and appropriately
positioning ground rods can minimize the possible risk of
shock for grid equipment and personnel. Furthermore,
burying conductors at an adequate depth and properly
grounding electric equipment with neutral point grounding
protects costly equipment and ensures the safety of
personnel.
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